A projection objective is part of a projection exposure tool for microlithography which is used to produce semiconductor components. For this purpose a pattern, called a reticle, disposed in an object plane of the projection objective, is imaged by the projection objective onto a photo-sensitive layer of a substrate which is called a wafer.
Due to the constantly progressing miniaturization of the structures of the semiconductor components to be produced, increasingly more stringent desired properties are made of the imaging properties of projection objectives. This typically involves reducing imaging errors of projection objectives for microlithography to a very low level. While production related imaging errors in a projection objective can already be eliminated after production of the projection objective by post-processing (for example, aspherization of individual lenses or mirrors of the projection objective), the correction of imaging errors occurring during operation of the projection exposure tool is more difficult.
During operation, the imaging radiation of the optical elements of the projection objective used is partially absorbed, and this leads to heating of the optical elements. By thermal expansion and, if applicable, associated refractive index changes, imaging errors which can take on complicated field characteristics are induced, in particular when, as is the case with modern projection exposure tools, the beam path through the projection objective is not rotationally symmetrical relative to a central axis, and in particular individual optical elements from the beam path are only used in a partial region.
Moreover, special illumination configurations (also called illumination settings) are being used more and more in modern projection exposure tools, in particular dipole or quadrupole illuminations. These multipole illuminations lead in particular to imaging errors or higher waviness or to imaging errors in higher Zernike orders.
Usually, in order to compensate heat-induced imaging errors occurring during operation, the projection objective is provided with an optical correction system. For example, an optical correction system is known from EP 0 851 304 A2 which includes two optical correction elements which both respectively have an aspherical surface contour on their surfaces facing towards one another. The two aspherical surface contours adding up to at least approximately, zero. This type of correction system is also called an Alvarez manipulator.
With an Alvarez manipulator the two surfaces facing one another are arranged close to one another spatially, in particular in planes conjugate to one another. This type of correction system can be relatively complex because the correction elements are provided in addition to the optical elements of the optical system. Furthermore, the correction elements lead to losses in intensity. In particular with so-called free-form surface designs (in which the optical system is provided with optical elements which have non-rotationally symmetrical surfaces) providing the aforementioned correction system may be impracticable, as with free-form surface designs the optical elements are matched to one another such that the asphericity of the latter is cancelled out overall.